Ribonucleotide reductase is a highly regulated enzyme involved in the DNA synthesis pathway. It is responsible for the de novo conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates that are essential for DNA synthesis and repair (Cory and Sato, (1983) Mol Cell Biochem 53, 257–266; and Thelander and Berg (1986) Mol Cell Biol 6, 3433–3442). Ribonucleotide reductase consists of two subunits, M1 and M2. M1 is a 170 kDa dimer required for enzyme regulation (Chang and Cheng (1979) Cancer Res 39, 5081–5086). M2 is an 88 kDa dimer containing a tyrosine free radical and a non-heme iron.